Caught black-marketeering in Berlin, Harry Palmer faces eight years in a military prison, unless the mysterious Major Dalby can find a use for his skills.

Harry and Jean arrive in Beirut in hot pursuit of Randall, who is holding Professor Dawson. Meanwhile, Dalby holds a secret rendezvous with Stok, a Russian colonel.

Dalby challenges General Cathcart, suspecting American involvement in Dawson's abduction. Harry and Jean head to the Pacific atoll where the neutron bomb is to be tested.

Harry is handed over to the Chinese military where he is subjected to a bewildering regime of mental torture. Dalby's love for Dr Lavotchkin takes its toll on his judgement.

Harry is tormented by what has been planted in his mind as his mission reaches its bloody climax and the stability of the free world is at stake.